As for "100% steel", that's a great idea, but putting it into practice may prove a dealbreaker. Depends on flux, coke/charcoal, how much iron ore deposits (a few magnetite clusters would help) you have on your map, how aggressive the goblinite delivery program is (and how well you handle it), and how much time you want to spend.
I do think a magma-reservoir main weapon for planetary bombardment is critical. A double-reservoir system for measuring out single "pulses" of fiery death. (Which means at least
some bauxite, one way or another.)
Hey, your saying a sphere of 24 diameter isn't impressive?
Meh, not "mega" quality impressive, no. I mean, as a first effort, hey, great - but a DS should be BIG - bigger than BIG!
And since it's round, it won't have a big usable internal footprint to start with and a lot of wasted space where square workshops don't fit into round sides - and significantly smaller and tighter curves on every level above/below the equator. If you're going to house a self-sustaining fortress within it, that gets cramped fast. (Altho' maybe I'm wrong - almost 24 levels is more than most fortresses use. That, and it'd be hard to find a map with more open levels above the ground. I might be envisioning it wrong.)
I'd play with your graphics program, see what bit-map the various sized circles work out to - that's both the top view of your equatorial level and the side-view curvature of the walls. You want something smooth, and larger is smoother. I'd think something around 30 (which is over 50% bigger), or even more. A magma vent averages about 30 across - that doesn't seem too big.
But otoh, it gets tougher in a cubing function the bigger you make it. Your call.